1. Field of the Invention
This invention relates to an indicator device, more particularly a rapid response device for the detection of carbon dioxide in a gas mixture.
2. Description of the Background Art
Devices for the detection of carbon dioxide which rely in part upon the change in color of certain chemical compounds according to the pH of their environment are known in the art. Such chemical indicators change color in solution when the pH of the solution changes.
Numerous examples of chemical indicators which are pH-sensitive, and thereby useful in carbon dioxide indicator systems, have been disclosed in the prior art.
U.S. Pat. No. 2,890,177 discloses a liquid chemical indicator for detecting the presence of carbon dioxide in respiratory gases comprising an aqueous solution of an alkali metal oxalate and a pH sensitive dye which changes color at a pH in the range of 6.6 to 5.8.
U.S Pat. No. 3,068,073 discloses a method for determining carbon dioxide in a gas which comprises passing the gas to be tested through a solid reagent comprising activated alumina carrying thymol blue and, optionally, a base.
U.S. Pat. No. 3,114,610 discloses a continous sampling gas analyzer comprising a pH sensitive dye suspended in a gel substance, a semi-permeable membrane which separates the gas to be tested from the dye but allows gas to pass therebetween for analysis, a light source for illuminating the dye and a detector for analyzing the light transmitted through said dye. For carbon dioxide determination the dyes disclosed are Methyl Red and Bromocresol Green.
However, none of the above patents directly addresses the problem of determining accurately and rapidly the correct positioning of an endotracheal catheter in the trachea of an apneic patient.
Introduction of a catheter in the trachea of a human may be required for a number of reasons. For example, in a hospital, an endotracheal catheter, also known as an intratracheal catheter, may be used for general anesthesia; in the field, a doctor or paramedic may use an endotracheal catheter to resuscitate an apneic patient. In both of these instances, and others, it is critical that the catheter be properly placed in the trachea and not, for example, in the esophagus. If the catheter is improperly placed and the error is not discovered within a very short time, on the order of 5 to 20 seconds, the patient may begin to suffer irreparable harm or even death.
In view of the criticality of the timing when an endotracheal catheter is improperly placed in an apneic patient, there is clearly a need for a simple device which will rapidly and reliably give an indication of improper (or proper) placement. (See P. K. Birmingham et al., "Esophageal Intubation", ANESTH ANALG, 1986, 65, 886-91).
One device for determining the correct location of an endotracheal tube is describe in U.S. Pat. No. 4,691,701 to Williams. In Williams the detector includes a housing and an indicator in the form of a transparent disc which covers an aperture in the housing. The disc has a chemical substance which provides a color change indication when exposed to carbon dioxide from the patient.
Another device for determining the correct placement of an endotracheal tube is described in U.S. Pat. No. 4,728,499 to Fehder. The device in Fehder comprises an enclosure defined by walls and having a transparent window in a wall, an inlet, an outlet and sealing means, the enclosure having mounted therein an indicator component adapted to be viewed through said transparent window. The component comprises a carrier having attached thereto an indicating element formed for (1) an aqueous solution of a colorless compound, i.e., a base, which provides an alkaline solution; (2) a hygroscopic, high boiling, transparent, colorless, water-miscible liquid; and (3) a chemogenic pH-sensitive indicator which changes color relative to a change in pH of the solution and which has a pH which is lower by 1.0-1.5 pH units than the pH of the solution. The nature and concentration of the colorless compound in (1) is correlated to the nature and concentration of indicator (3) so that no color change occurs for at least 15 minutes when the indicating element is exposed to an atmosphere having a concentration of 0.03% carbon dioxide, but a color change is produced within 5 to 10 seconds, when the indicating element is exposed to an atmosphere containing at least 2% carbon dioxide.
A carrier particularly preferred by Fehder is a thin layer of bibulous material, such as filter paper or fibrous synthetic material, and the indicating component is formed by impregnating the bibulous material with the indicating element and drying to remove excess moisture.
However, a deficiency common to the above two devices is that they are composed of small parts which must be assembled prior to use. Further, because of their structures, they are designed to be used at the end of an endotracheal tube and not inside an endotracheal tube. There therefore exists a need for a carbon dioxide indicator which is structurally simple, easy to assemble and capable of being used anywhere in a respiratory circuit, such as inside an endotracheal or other tube in the circuit, or inside a connector which joins tubes in the circuit.